Stress relieving apparatus



Jan. 27, 1953 c. A. MAXWELL ETAL STRESS RELIEZVING APPARATUS Original Filed Oct. 5, 1944 3 Sheets-Sheet z X EL N P- R EN Jan. '27, 1953 c. A. MAXWELL ETAL. 2,625,792

STRESS RELIEVING APPARATUS O riginal Filed Oct. 5, 1944 3 Sheets-Sheet 3 CARL A MAXWELL JOBN P QRAVBELN Patented Jan. 27, 1953 STRESS RELIEVING APPARATUS Carl A. Maxwell and John P. Craven, Akron, Ohio, assignors to The Babcock & Wilcox Company, Rockleigh, N. J a corporation of New Jersey Original application October 5, 1944, Serial No. 557,328. Divided and this application May 26, 1948, Serial No. 29,225

2 Claims.

This invention relates to apparatus for effecting the stress-relieving heat treatment of welded pressure vessels.

This i a divisional application of our parent application Serial No. 557,328, filed October 5, 1944, now Patent No. 2,464,248.

The invention is more particularly concerned with the stress-relieving heat treatment of large pressure vessels after they are in operative position.

Many manufacturing processes in the field of industrial chemistry require welded pressure vessels of large size. This characteristic, together with the fact that many of these vessels are so constructed and set so that they tower to considerable heights, demands a mode of erection involving the welding together of sections of the vessel in the field. This procedure makes subsequent stress-relieving difiicult and eliminates the use of a fixed stress-relieving furnace in which pressure vessels may be treated in seriatim.

Furthermore, the expense of building a large knock-down stress-relieving furnace towering more than 100 feet above the earth, and for erection about different pressure vessels, successively, would be prohibitive.

The general object of this invention is to overcome these difiiculties, providing novel apparatus for stress-relieving pressure vessels in the field.

A further object of the invention is to accomplish the field stress-relieving of pressure vessels while maintaining the time-temperature and temperature gradient schedule limitations of the particular stress-relieving procedure imposed by the characteristics of the pressure vessel metal.

The invention will be described with reference to the operation and construction of the apparatus shown in the accompanying drawings, and other objects of the invention will appear as the description proceeds.

In the drawings:

Fig. 1 is an elevation showing the apparatus in operative relation to a pressure vessel to be stress-relieved;

Fig. 2 is a plan of the apparatus shown in Fig. 1;

Fig. 3 is a view showing the furnace in vertical section, and indicating the relationship of the furnace and the circulating fan;

Fig. 4 is a plan of the furnace and circulating fan unit shown in Fig. 3;

Fig. 5 is a plan of the reversing damper construction;

Fig. 6 is a transverse vertical section on the line 6-6 of Fig. 5, showing the structure of the reversing valve or damper; and

Fig. 7 is a fragmentary section of the reversing damper taken on the line l-l of Fig. 6, and looking into the direction of the arrows.

The pressure vessel ll) of Fig. l is of a type used in the manufacture of one of the petroleum derivatives used in the production of synthetic rubber. Its diameter is of the order of 15 feet; its wall construction is of steel plate of a thickness of the order of two inches; and the height of the pressure vessel itself is in excess of feet. With the height of the skirt [2 and base I4 added, the total height of the pressure vessel installation is in the range of -200 feet. The erection of such pressure vessels in the field necessitates the joining of sections by welding, and this procedure necessitates subsequent stress-relieving of the pressure vessel before its use under condition involving substantial temperatures or pressures or both.

For such stress-relieving, our invention in volves a large diameter gas distributing duct I6, temporarily fixed centrally of the pressure vessel, and the duct l6 has openings distributed along its length to cause a distribution of some of the incoming heating gases along the length of the pressure vessel.

The top of the duct I6 terminates at a position substantially spaced from the top of the pressure vessel but near enough thereto to prevent stagnation of gases in the top of the pressure vessel during the circulation of the stress-relieving gas.

Associated with the lower end of this duct l8 there is piping or duct-work extending through a nozzle or other opening 24 in the bottom of the pressure vessel and connected to the gas outlet of a furnace 22. This duct work involves a T 26 disposed above the furnace as indicated in Fig. 1. This construction conveniently provides for an explosion door 28 at the top of the T 26, and also provides for necessary access to the furnace for inspection or repair.

From the outlet of the T 26, duct sections 30-33 lead to the port 36 of the reversing damper construction indicated generally at 40 in Fig. 5.

With the reversing damper 42 set by its manually operable controller 44 in the position indicated in Fig. 5, the flow of gases will be from the port 36 to the port 45 and thence through the duct sections 50-58 through to the inlet of the distributing duct (6.

As the gases flow upwardly in the duct 16, a part of the gases (for example 25%) is distributed along the length of the pressure vessel through the openings 6| in the duct, but the major part of the gases passes through the duct to the upper end of the pressure vessel. The gases then turn and pass downwardly between the duct I6 and the pressure vessel walls, to an opening 60 connected by the duct sections 65-68 with the port ID of the reversing damper construction. The gas flow continues through the reversing damper and out through the port 12, which is directly connected to the inlet 14 of the circulating fan I6 driven by the motor I8.

When the gases move through the gas circuit in the manner described, heat transfer takes place through the metallic walls of the duct IS. The gases are at their highest temperature near the inlet of the duct, and as the gas flow continues, the tendency is for the metal in this zone to obtain an undesirably high temperature. This tendency is alleviated or compensated by the transmission of heat to the duct metal in the gas inlet zone, and the retransmission of heat from that metal to the shell of the pressure vessel and to the cooler gases exteriorly of the duct and near the gas outlet zone. This action, in combination with the gas flow outwardly through the openings in theduct results in increased temperature of the gases near the gas outlet zone and produces a more uniform gas temperature at positions spaced longitudinally of the shell of the pressure vessel.

When the direction of gas circulation is reversed the duct walls near the gas inlet will act totransmit heat to the gases approaching the outlet by reason of the fact that the duct at this position is surrounded by the inlet gases at high temperature. In this case, the absorption of heat by the duct walls from the surrounding gases tends to equalize gas temperatures, preventing excessively high metal temperatures in the zone of the gas inlet. These temperature equalizing actions improve the stress-relieving process, and reduce the frequency of gas flow reversals.

The outlet of the circulating fan I6 is directly connected to the inlet 80 of the outer furnace casing 82, which is shaped as indicated in Fig. 4. The circular combustion chamber 86 is disposed within the outer furnace casing 82 so as to distribute the incoming circulating gases around the combustion chamber through the gradually restricted passage 88 between the outer wall of the casing 82 and the combustion chamber 86. The mixed circulating gases, and makeup gases from the combustion chamber 85 pass upwardly to the furnace outlet 20, and the circulation of the gases continues in this manner until the burner BI) is shut off or until the damper 42 is moved by its controller 44 to a position at right angles to the position in which it is shown in Fig. 5. When this is done, the flow of the heating gases is reversed. Then thefiow is from the reversing damper port I through the duct sections 68, 61, 66, 65, to the lower part of the pressure vessel II] and then upwardly along the walls of the pressure vessel exteriorly of the duct I6 to the upper end of the pressure vessel. The gases then flow downwardly through the duct and thence to the duct sections B58 to the port 45 of the reversing damper construction. From this port, the flow is through the port 12 and to the inlet of the circulating fan 16. The flow of gases is reversed intermittently during stress-relieving procedure to keep temperature gradients within predetermined limits, the temperatures of different sections of the pressure vessel being shown by thermo-couples disposed at spaced po 4 sitions on the vessel. Excessive temperature conditions around the gas inlet and outlet openings 24 and 60 are particularly avoided by this action.

The combustion chamber 86 is preferably lined with ceramicrefractorymaterial and has a ceramic refractory ring 92 around the burner 90. The latter is preferably an oil burner, supplied with primary air by the fan 94.

The construction of the reversing damper assembly is indicated in Fig. 6 of the drawings. It comprises a steel casing I00, having central bearing parts I02 and I04, in which the tube I06 is turnably mounted, with the controller 44 fixed at one end as indicated in Fig. 6. The tube I06 is divided by a central diaphragm I08 into inlet and outlet chambers III) and H2. These chambers are connected by U-tubes II4--II6, which are nested as indicated in the drawing. They are also provided with extended surface elements I2Il-I25, disposed as indicated in Figs. 6 and 7. These elements are effective to mechanically support and thermally maintain the heat resisting refractory material I30, which is pressure molded around them to complete the damper blade. The inlet and outlet chambers III] and H2 are similarly connected by the U-tubes disposed on their diametrically opposite sides, the tubes being similarly constructed and associated with refractory material to afford the frame work for oppositely extending blades.

At one end of the reversing tube I06 is an inlet I40 for fluid which flows into chamber H0 and thence through the U-tubes to the outlet chamber I I2. The fiuid passes from this chamber to the outlet I I5, and the circulation is maintained to prevent overheating of the damper construction. Overheating of the casing I00 is also prevented by a refractory lining I50, molded against the walls of the casing and having an interior surface closely adjacent to the periphery of the damper.

In the stress-relieving operation, excessive temperature differentials in the metal of the pressure vessel must be avoided, and for this purpose, as well as the control of the temperature changes, the pressure vessel is covered with thermal insulating material before the stressrelieving procedure is begun. The skirt I2, supporting the pressure vessel, is also enclosed with insulation. This insulation may consist of a rock wall blanket securely held against the metal surfaces.

In an illustrative stress-relieving procedure, the pressure vessel is heated slowly and held at a temperature of a range of 1100-1150 F. for a twelve hour soaking period. During such a period, the gases leaving the reversing damper construction may be of the order of 1300 degrees R, and of the order of 1100 degrees F., returning. During both the heating-up and the soaking period, heat increments and gas flow reversals are provided in accordance with the temperatures indicated by thermocouples disposed at various positions on the pressure vessel. The soaking period varies with the wall thickness of the vessel as well as other factors and is usually of several hours duration. Heat losses during this period, due to gas leakage or other causes, are compensated by intermittent firing of the furnace.

The cooling period of the procedure is then started, and temperatures are lowered at a rate of 30 degrees per hour, flow of the gases being intermittently reversed through the pressurevessel and the rate of furnace firing being varied to control the cooling. During this cooling, the temperature differential between any two points in the vessel is held below 100 degrees F. The temperature is controlled until all points in the pressure vessel are below 300 degrees F. At this time, the gas flow connections, such as those at 24 and 60, at the lower end of the pressure vessel, are removed, and these openings, as well as any others in the pressure vessel, are closed and pressure sealed. When all temperatures are below 160 degrees F., water at 160 degrees F. is placed in the vessel, and hydrostatic test pressure is applied. The hydrostatic test has proven satisfactory to 340 lbs. p. s. i., this pressure being held for four hours.

While in accordance with the provisions of the statutes we have illustrated and described herein the best form of our invention now known to us, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by our claims, and that certain features of our invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In apparatus of the class described, a pressure vessel of great height erected by field welding, said pressure vessel having openings near its base providing for a heating gas inlet and a heating gas outlet, a blanket of thermal insulation covering the pressure vessel externally and secured thereto, a duct in the nature of a standpipe temporarily secured centrally within the pressure vessel and extending from the zone of said openings to a position near the top of the vessel, a furnace including a combustion chamber and a burner, an exterior chamber arranged about the combustion chamber for the circulation of heating gases around and past the combustion chamber, a gas circulating fan having its outlet connected to said exterior chamber, a reversing damper casing having four ports, one of said ports being connected with the intake of said circulating fan, duct work connecting the outlet of said furnace with one of said ports opposite the inlet of the circulating fan and conducting the heating gases from the furnace to the reversing damper casing, a second set of duct work extending through said gas inlet opening of the pressure vessel and connecting the lower end of said internal duct with one of the remaining ports of said reversing damper casing, a third set of duct work connecting said gas outlet opening of the pressure vessel with the remaining port of the reversing damper casing, a fluid-cooled reversing damper within said casing, means for turning said damper to cause reversal of the flow of gases through the pressure vessel and the duct work system.

2. In apparatus of the class described, a pressure vessel of great height erected by field Welding, said pressure vessel having openings near its base providing for a heating gas inlet and a heating gas outlet permitting a substantial flow of heating gases through the vesssel, a blanket of thermal insulation covering the pressure vessel externally and secured thereto, an upright duct temporarily secured centrally within the pressure vessel and extending from the zone of said openings to a position near the top of the vessel, said upright duct being open to the interior of the vessel at its upper end, a furnace including a combustion chamber and a burner, means presenting a gas heating exterior chamber around the combustion chamber for the circulation of heating gases around and past the combustion chamber, the exterior chamber freely communicating with the gas outlet of the combustion chamber, a gas circulating fan having its outlet connected to said exterior chamber, a reversing damper casing having four ports, one of said ports being directly connected with the intake of said circulating fan, duct work connecting the outlet of said furnace with another of said ports opposite the inlet of the circulating fan and conducting the heating gases from the furnace to the reversing damper casing, a second set of duct work extending through said gas inlet opening of the pressure vessel and connecting the lower end of said internal duct with one of the remaining ports of said reversing damper casing, a third set of duct work connecting said gas outlet opening of the pressure vessel with the remaining port of the reversing damper casing, a fluid-cooled reversing damper within said casing, means for turning said damper to cause reversal of the flow of gases through the pressure vessel and the duct work system.

CARL A. MAXWELL. JOHN P. CRAVEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 291,967 Webb Jan. 15, 1884 648,259 Hollingsworth Apr. 24., 1900 655,092 Kopping July 13, 1900 927,378 Palmer July 6, 1909 927,930 Von Bechen July 13, 1909 1,069,802 Mokry Aug. 12, 1913 1,445,220 Lee Feb. 13, 1923 1,933,571 Trinks Nov. 7, 1933 2,057,841 Newhouse Oct. 20, 1936 2,160,633 Young May 30, 1939 2,464,248 Maxwell et a1 Mar. 15, 1949 

